text:
When/How may I replace the Turn Coordinator with an Artificial Horizon?
I've seen a few panel shots where the traditional turn coordinator has been replaced with a second artificial horizon, as shown below.

The benefits seem obvious - if my artificial horizon failed I'd certainly like to have a fully functional back-up rather than a turn coordinator with no pitch information.
Under what circumstances/conditions are these second horizon gyros acceptable as replacements?
Are there any drawbacks that I should be aware of if I'm considering installing one.

In an inverted flat spin the turn coordinator is the only one that will give you a good idea which way the a/c is spinning so you can use the correct rudder to stop. Artificial horizon or DI will not give you correct readings. Stay out of inverted flat spins and you should be fine.

@Radu094 the turn coordinator is only accurate about 50% of the time in any kind of spin. It shouldn't be relied on to get you out.

well, my comment was more tongue in cheek, but 50% seems a bit too low a reliability. Can you point to any further sources why the coordinator shouldn't be trusted in a spin (especially in an upright one where I was living under the impression that it is always correct)

@StallSpin: Are you sure you don't mix up artificial horizon and rate of turn? It's the artificial horizon that is often wrong in a spin, because it can suffer gimbal lock and because it's calibration mechanism actually decalibrates it in extreme manoeuvres. Rate of turn has neither gimbals nor calibration and should not suffer any problems.

FYI most people who replace the TC in the six pack with a backup AI at least keep the TC mounted somewhere else on the panel. You do want to be able to make a standard rate turn still, especially IFR, because that is what ATC is expecting.

@Radu094
Your BALL is subject to centripetal forces when you enter the spin. Its location in the aircraft relative to the center of mass will almost always determine which direction it deflects, and it almost always deflects to the outside of the spin (a ball on the left will deflect left, a ball on the right will deflect right).
I don't know if all or any gyro RoT indicators will function correctly in a spin, I've always been under the impression that they will also be subject to side forces depending on where they are mounted.

@StallSpin: The slip-skid ball will be useless, definitely. But that is not the instrument we are discussing. Gyros however don't respond to centripetal forces, but to rotation. Reading of rate-of-turn gyro will be always correct (as long as it works at all), reading of free gyro (attitude indicator) will only be correct if gimal lock does not occur. For simple gimball that means there is pitch and bank limit beyond which it won't show correctly. It's more likely to be exceeded in spiral dive where the bank is higher.

@StallSpin; just to nit pick, because of this "there's no centrifugal force" pet peeve of mine. The ball is only subject to centripetal forces due to contact with the glass in the deflected position (keeping the ball from exiting the plane...), the glass holding the ball is subject to centripetal forces though, dragging it away from the ball causing it to look deflected. In the spinning (i.e. constantly accelerating) reference system that is the cockpit, the ball is actually deflected by the centrifugal force. That, or as opposed to the rest of the plane, not subject to centripetal force.

@JanHudec My mistake, I picked up a habit somewhere of only calling the slip/skid a turn coordinator, not including the rate of turn instrument.
Roe - I'm with you on this one, and I typed centrifugal but my spell check doesn't like it.

@StallSpin I think most pilots are guilty of using the shorthand "Turn Coordinator" to refer to either of the components when in reality one rate-of-turn indicator + one slip-skid (balance) indicator = one "Turn Coordinator". Generally not a problem since other pilots can usually figure out what part of the instrument you intend them to be looking at :)

We call it an inclinometer here in the United States. (I know a bunch you guys are UK or EU) I have heard slip/skid indicator as well though, and I am also familiar with the ball.

@roe: General relativity rehabilitated the centrifugal force. According to general relativity inertial reference frames are no longer special, so centrifugal force is completely equivalent description of the same reality from perspective of the rotating reference frame. Describing it as centri fugal force pushing the ball to the side of the instrument rather than as centripetal force pushing the instrument from the ball is clearer in this case (the rest of the nit still applies).

@Jan Hudec: diverging from aviation topic, but please re-read your general relativity handbook. Rotating reference frames are not included.

Check out AC 91-75.
I believe it has everything you're looking for. It seems the FAA really doesn't like turn coordinators these days:

“[T]he FAA believes, and
  all other commenters apparently agree … the rate-of-turn indicator is no
  longer as useful as an instrument which gives both horizontal and vertical
  attitude information.”

Which is completely true...
I don't see a drawback, really. You'll also have to buy a slip-skid indicator (it seems that most replacement attitude indicators can be ordered bundled with one.), so you're really not losing anything.
Edit: The other posts and comments raise some good discussion, I suggest reading those as well.

Interesting that in all the FAA's talk about how awesome it is to have a second horizon they never bring up the gimbal lock issue.

If you get in a spin in IMC, you should rely primarily on the airspeed indicator and the rate-of-turn gyro. The inclinometer ball cannot be trusted; it is likely to be centrifuged away from the center of the airplane — giving an indication that depends on where the instrument is installed, telling you nothing about the direction of spin. The artificial horizon (attitude indicator) cannot be trusted since it may have tumbled due to gimbal lock. It is better to trust the rate-of-turn, which cannot suffer from gimbal lock, it has no gimbals. (Remember, it is a rate gyro not a free gyro)

The How It Flies in section on Spiral Dive Recovery mentions:

If you don’t have good outside references, you should not rely on the attitude indicator (artificial horizon). The attitude indicator contains a gyro mounted on ordinary mortal gimbals, which can only accommodate a limited range of pitch and bank angles. A steep spiral can easily cause the gyro to tumble, whereupon it will need several minutes of relatively straight and level flying before it can re-erect itself. Military aircraft have non-tumbling attitude indicators, but you’re not likely to find such things in a rented Skyhawk. Therefore, you should roll the wings level by reference to the rate-of-turn gyro.8 Being a rate gyro (as opposed to a free gyro) it has no gimbals, and therefore can’t possibly suffer from gimbal lock.
8 That is, the turn needle or turn coordinator, whichever you happen to have.

And in section on Spin Recovery it adds:

If you get into a spin in instrument conditions, you should rely primarily on the airspeed indicator and the rate-of-turn gyro. The inclinometer ball cannot be trusted; it is likely to be centrifuged away from the center of the airplane — giving an indication that depends on where the instrument is installed, telling you nothing about the direction of spin. The artificial horizon (attitude indicator) cannot be trusted since it may have tumbled due to gimbal lock. It is better to trust the rate-of-turn, which cannot possibly suffer from gimbal lock, since it has no gimbals. Remember, it is a rate gyro (not a free gyro), so it doesn’t need gimbals.

If you have good enough artificial horizon designed to avoid gimbal-lock, the you don't need turn coordinator. But with ordinary artificial horizon the turn coordinator combined with vertical speed indicator (despite it's problems like rather slow reaction time) are useful backup for recovering from in-flight upsets.

Interesting point about driving the mechanical artificial horizon into gimbal lock with aggressive maneuvering. I wonder if they've eliminated this issue with solid-state (MEMS gyroscope) horizons (since those are essentially three single-axis gyros similar to the rate-of-turn gyro). A subject for a future question :-)

@voretaq7: If the system is made of solid state rate gyros with integration by computer, it can't suffer gimbal lock, because it does not have any gimbal. Plus the solid state gyroscopes with good integrator are actually more precise than mechanical ones. I suppose it's part of a reason why glass cockpit suites generally don't feature turn coordinator any more.

@JanHudec; you'd be amazed how often software is poorly written. Gimbal lock in software is quite possible. :) Although I'd have to hope that a developer of actual aircraft software would know what Gimbal lock is, and not end up in it. Didn't a space shuttle have an issue with Gimbal lock in software once..?

@roe: I know well how much software out there is utter crap. But the integration code is not that big and if it does not correctly combine the angles, it should manifest progressively rather than only beyond certain angle as for gimbals, so it should be easier to discover in testing.

@JanHudec Mathematically it depends on if they're using Euler angles internally, or deriving them later (e.g. from quaternations) - the former uses cheaper hardware and is easier to code, but suffers from gimbal lock issues when axes cross their 90-degree points ; the latter requires "extra" gyros (more expensive hardware) and some math (more time writing software), but is virtually impossible to gimbal lock.

I can think of a couple of things, the most prominent is the power source. Turn indicators are usually electrically powered, whereas attitude indicators are vacuum powered, and the idea is to provide redundancy should either power source fail. This is not always the case though, and if your second attitude indicator is powered electrically, that's obviously a non-issue.
The second thing is the absence of a ROT indication, forcing you to calculate the correct bank angle for a standard rate turn should you ever really need one. The only situation I can think of where it is imperative that you can do a standard rate/half rate turn is a no-gyro approach in which case you've probably also lost your vacuum source, and you don't want to start playing mind games, but it's not as crucial as the power source redundancy.
As for whether it's acceptable, StallSpin seems to have that covered. The pictured second attitude indicator is fitted with a slip indicator, so that should cover that aspect.

I agree with this, the ROT is much better to trust (as far as instruments) in a spin. Also there are times in IFR especially in a no-gyro situation that it becomes useful. I have had a gyro failure in actual IMC and knowing standard rate was crucial for the no-gyro turns ATC was giving me. The plane I was flying actually had an electric AI with an inclinometer (castleberry), but we kept the TC on the co-pilot side. Best of both worlds it seems.

The turn coordinator or turn and slip indicator (they are different) usually are electric powered.  The artificial horizon is usually vacuum driven.  If you replace the turn coordinator with an artificial horizon, it should be electric, assuming the existing one is vacuum.
Rate gyros are not prone to gimbal lock, or "toppling", as are most attitude indicators.  If an attitude indicator topples, it can take several minutes to correct.
Also, read 91.205.  Generally, a gyroscopic rate-of-turn indicator (needle) and a slip-skid indicator (ball) are required for IFR flight.  Two attitude indicators won't do.

Be careful with blanket pronouncements: two attitude indicators in lieu of a gyroscopic rate-of-turn indicator (needle) is perfectly acceptable for IFR flight in a pretty large chunk of the GA fleet (per the previously-referenced AC two attitude indicators can be an Administrator-approved method of compliance with the requirement to have a gyroscopic rate-of-turn indicator specified by section 91.205(d)(3) if certain requirements are met).


text:
Why can not two modes coincide perfectly for some cases in Flutter phenomena?
I have a question about flutter phenomena. As you know, flutter occurs when two modes (for example, first mode and second mode) coincide (that is, after a specific velocity, the frequencies of two modes will be the same and equal) and the damping term will change into positive which causes failure.

However, for some cases, frequencies of two modes do not coincide perfectly (they are not equal) and damping parameter negative sign will change into positive, which mean flutter is happening.

Why is this happening? Why can't the frequencies of two modes coincide perfectly?

Not two modes of the same degree of freedom converge (1st and 2nd mode are harmonics, so their frequency ratio is fixed), but flutter occurs when the frequencies of two different oscillations come close. They do not need to become identical, just close enough that they build up each other's amplitude.

@ Peter Kämpf: Thank you for your great help. I want to know why in some cases these 2 modes come close, and in some cases they become exactly identical.

Wouldn't this be, perhaps, more likely to be answered over at http://physics.stackexchange.com/?

@Jan: Sure, but this goes for all questions about flight mechanics, aerodynamics and dynamics. This question is in the field of aeroelastics, and appropriately enough, it carries this tag.

@Shellp in your second graph have you performed a mode tracking to be sure they didn't cross at the flutter point?

Flutter happens when the frequency of two modes coincide. These modes must be of different nature, so their frequencies can move in different directions. Typical examples are elastic modes (with eigenfrequencies independent of speed) and aerodynamic modes (with eigenfrequencies proportional to speed). When flight speed increases, the aerodynamic mode becomes faster (think of dynamic pressure as of the stiffness of the spring in a spring-mass system) while the elastic mode stays constant. At some point both have the same frequency, but flutter will already appear when they are close enough so each builds up the amplitude of the other. This will become more efficient when the frequencies are identical, but this is academic:  When flutter starts, it is a bad idea to accelerate further.

Example for an elastic mode: Wing bending. The first mode is just the tips moving up and down, the second mode is one tip up, the other down, which will rotate the fuselage, the third mode (or for pedants: The second symmetric mode) is again both tips up, but now with the midspan section moving down and so on. They are harmonics (like the oscillations of guitar strings), so their frequencies are at a fixed ratio, with the first mode having the lowest frequency. See below, from top to bottom.

Example for an aerodynamic mode: Aileron flutter, fast-period mode (especially when coupling to the bending mode in sweptback flying wings).

For me, your diagrams don't make sense. They should look more like that:

EDIT:

With Shellp's comments I realized that the answer above is too simple. The lines in the graph are for the static case of the elastic vibration. Once this wing moves through the air, it's movement will change the local aerodynamic forces, which will modify the flutter modes. The motion-induced aerodynamic forces counteract the elastic motion at slow speeds, adding damping (negative, because they reduce the motion). With damping, the frequency of the mode is reduced.

At higher speeds, delays in the buildup of pressures mean that now the aerodynamic forces lag behind the motion, and once this lag exceeds a quarter of a period, they produce positive damping (this is an unfortunate wording, so stay with me. The damping term is a negative number when it really damps the motion, but when the term becomes positive at higher speeds, it is still called damping, but now the motion is excited instead of damped in the common sense. So let's call positive damping "excitation" from now on).
This excitation now increases the flutter frequency, so in reality the two flutter phenomena now get locked together. The first graph in the question above shows the frequency going down with speed, which looks unusual to me. The second graph shows this locking-in with an increase in frequency over speed, but at very high frequencies (it must be a small structure). If you want, the excitation is pushing the elastic frequency up, so both lock together without converging completely. But that cannot last forever - at higher speeds they should diverge, but in reality the structure is overloaded way before that happens.

Thank you Peter! My diagrams are correct[ If you still have doubt, I can send you several papers about flutter topic! ;) ]. The behavior of the V(Flight Speed)-w( Frequency) diagram, depends on which method you use to calculate the Flutter(p,k,p-k,g and...methods). But all in all, the point is that 2 frequencies become identical and damping parameter's sign will change into positive sign.

I have read and seen in many papers the diagrams like my second figure that I mentioned above, but none of them mentioned that why for some cases these two frequencies do not become identical and keep distance!
I would be grateful if you could help me! :)

@Shellp: This could be due to nonlinear effects. If the amplitude is big enough, the simple assumptions in my answer do not hold, and both the elastic and the aerodynamic behavior change. See the SB-9 flutter movie (http://www.youtube.com/watch?v=RenbFgLZBNA) - normally the flutter would grow bigger and quickly destroy the plane. Nonlinearity helped to limit the flutter amplitude. Concerning damping: Positive damping means that now the oscillation will build up instead of dying down. That is another way of saying that flutter happens.

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text:
What restrictions apply to flying RC and UAV planes in India at very low altitudes?
We are working on a UAV and will soon get to the testing phase. We intend to fly at a maximum altitude of 300-400 meters. I wanted to know as to what restrictions apply to the airspace in India (specifically Delhi) with respect to flying UAVs as well as the altitudes from which they apply?
We will also be testing autonomous capabilities. Do we need to get any licenses for this?

I'm not sure we have anyone familiar with India's airspace rules here (yet) - but either way you'd probably want to contact someone from the Directorate General of Civil Aviation which is your regional regulatory body. As much as you might get good advice here, you really want to be sure you're operating within the limits of appropriate regulation, and because UAVs are still "new territory" in most countries it's often helpful to talk to the regulatory agency directly.

Also, if you find your own answers via the DGCA, please come back here and answer your own question. It will help other people down the road, and is good for the site.

The Director General of Civil Aviation has published this public notice:

GOVERNMENT OF INDIA OFFICE OF THE DIRECTOR GENERAL OF CIVIL AVIATION
  OPPOSITE SAFDARJUNG AIRPORT, NEW DELHI – 110 003
PUBLIC NOTICE   File No. 05-13/2014-AED Dated: 7th October, 2014
Subject: Use of Unmanned Aerial Vehicle (UAV)/ Unmanned Aircraft
  Systems   (UAS) for Civil Applications
Of late, lots of interests are being shown for civil use (both
  commercial and recreational) of UAS in the country. International
  Civil Aviation Organization (ICAO) is yet to publish Standards And
  Recommended Practices (SARPs), as far as certification and operation
  of civil use of UAS is concerned.
UAS has potential for large number of civil applications. However, its
  use besides being a safety issue, also poses security threat. The
  Airspace over cities in India has high density of manned aircraft
  traffic. Due to lack of regulation, operating procedures/ standards
  and uncertainty of the technology, UAS poses threat for air collisions
  and accidents.
The civil operation of UAS will require approval from the Air
  Navigation Service provider, defence, Ministry of Home Affairs, and
  other concerned security agencies, besides the DGCA. DGCA is in the
  process of formulating the regulations (and globally harmonize those)
  for certification & operation for use of UAS in the Indian Civil
  Airspace. Till such regulations are issued, no non government agency,
  organization, or an individual will launch a UAS in Indian Civil
  Airspace for any purpose whatsoever. The above is for strict
  compliance.
Director General of Civil Aviation

Since no-one has answered this yet, I'll give it a crack.
According to the Unmanned Systems Association of India, UAVs are required to submit a specific request to the Directorate General of Civil Aviation to allow them to fly UAVs in a specific area.
For RC aircraft, you must make sure that they use one of the following frequencies (source):

27004 KHz
27116 KHz
27148 KHz
27156 KHz
27228 KHz

I'm not so sure about what makes an aircraft in the RC category, as opposed to an UAV though -- if someone could fill me in on that, that'd be great.

authors: Bob Jarvis - Слава Україні

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text:
Can I log instrument flight time for currency purposes in an Advanced Aviation Training Device (AATD)?
Advisory Circular AC 61-136 which outlines the approval and limitations of flight training devices seems to contradict 14 CFR 61.57(c).
To what extent can I use an AATD for instrument currency? In addition, to what extent can an AATD be used toward and instrument proficiency check (IPC)?
Edit:
I have located this document from the FAA Office of Aviation Safety that clarifies that AATDs and BATDs are governed by FAR 61.4(c) and are authorized for specific purposes and can be used in the same manner at a Flight Training Device instead of a Aviation Training Device as long as they have an approve Letter of Agreement (LOA) from the administrator.
This leads me to believe that an AATD qualifies for FAR 61.57(c)(2) not FAR 61.57(c)(3).

Can you explain a little bit more about how they are contradictory? Is it that the AC states a BATD can be certified for 61.57(c)(1), but 61.57(c)(3) is the part of the regs that addresses use of an ATD to log recent experience?

It would be great if you could quote the contradictory statements in your question.

FAR 61.57(c)(3) limits aviation training devices to 3 hours of flight time and adds requirements for unusual attitude recovery, while Appendix 3 section 2 of AC 61-136 pertaining to AATD authorized use simply states "Logging instrument flight experience". Also the AC in that same section authorizes up to 50 hours toward a Commercial certificate and 25 toward an ATP, while FAR 61.159(a)(3) states that an Aviation Training Device may not be used to satisfy the ATP requirements.

Appendix 3 section 2 of AC 61-136 pertaining to AATD authorized use also permits use towards and IPC minus circle to land while FAR 61.57(d)(1) says that an IPC must be in an aircraft.

Magnetoz, please edit your question (click the "edit" link just under it) to add more details.  It is much more readable and more people will see it as compared to comments.

@Magnetoz actually, 51.57(c)(3) doesn't limit ATD to 3 hours - it requires at least 3 hours in an ATD. That's an important distinction.

From what I can tell, the Advisory Circular is outdated. It was issued July 14, 2008, while the most recent update to 14 CFR 61.57 was effective Nov 15, 2013.
When the AC was issued, 61.57(c) did not contain the wording specific to Aviation Training Devices (effective 11/17/2003). That language was added in the version effective 10/20/2009, and the AC apparently has not been revised since that point.
At this point the regulations are more specific (and regulatory) than the Advisory Circular. You should be able to log recent flight instrument experience in a ATD based on 61.57(c)(3), or the subsequent subsections that pertain to mixed currency.

(3) Use of an aviation training device for maintaining instrument experience. Within the 2 calendar months preceding the month of the flight, that person performed and logged at least the following tasks, iterations, and time [emphasis mine] in an aviation training device and has performed the following--

(i) Three hours of instrument experience.
(ii) Holding procedures and tasks.
(iii) Six instrument approaches.
(iv) Two unusual attitude recoveries while in a descending, Vne airspeed condition and two unusual attitude recoveries while in an ascending, stall speed condition.
(v) Interception and tracking courses through the use of navigational electronic systems.

Egid, this question might help.

Forgot about that one. Thanks!

@egid - see my edit about this document I found.

I wrote this back in 2014 following a briefing on a Cardinal accident where two people perished on approach to CYOW (Ottawa). I should have posted it here back then.
At an IFR Club meeting on 27 Feb 2014 involving the accident of C-FEFQ, the issue of improperly logging “instrument time” for IFR recency was highlighted. I believe that commercial pilots, cruising along in VMC in Class A airspace where operations are IFR, log all their flight time as “instrument time” even when they are on autopilot and not actually operating the aircraft.  This is probably why many GA IFR pilots log their IFR time (in VMC) as “instrument time” as well.  Unfortunately, based on the Canadian CARs, GA IFR Pilots cruising along in VMC at 5000’ on an IFR flight plan cannot log their flight time as “instrument time” unless it is either actual flight instrument time, simulated flight instrument time or ground instrument time.  There seems to be a problem of interpretation in logging “instrument time” that no one wants to address. There is even a problem with the term "IFR Time" itself, which isn't your Flight Time. IFR Time really means your time in the IFR system. You could be in VMC for quite awhile before getting your clearance, then have your IFR clearance cancelled well before arriving at your destination. Your IFR Time would be less than your Air Time, an you never even see a cloud!
I believe this issue of logging “instrument time” has been debated for years and years (and years!) with lots of interpretations based on FARs Section §61.51- Pilot logbooks and §61.57- Recent flight experience: Pilot in command, as well as CARs Part IV - Personnel Licensing and Training, Section 401.05(3) - Recency Requirements and Section 401.08- Personal Logs.
Here is how I would recommend logging your “instrument time” for recency. In each situation you must be operating the aircraft or approved simulator solely by reference to the flight instruments:
Actual Flight instrument time

This is your time spent in cloud, smoke, fog, haze, mist or dark night conditions when you don’t have any horizon or lights for visual reference to the ground during an actual flight in non-VFR conditions.
If you are in these non-VFR conditions, you will obviously have to be on an IFR Flight Plan to be legal.
If you are in non-VFR conditions for the entire flight you can only take credit for up to the maximum Air Time ( up – down), not the entire Flight Time. You are not flying solely on instruments while taxiing.

Simulated Flight instrument time

This is your time spent simulating cloud, smoke, fog, haze, mist or dark night without an horizon or without visual reference to the ground during an actual flight and in VFR conditions.
You are expected to have a safety pilot with you and to use a view limiting device or hood in order to get credit for logging this simulated flight instrument time.
The safety pilot will also be your instructor when you are conducting IFR training in order to get credit for logging this simulated flight instrument time. You are allowed to log it, and your instructor (PIC) is also allowed to log it.
You can only take credit for the time that you are simulating instrument conditions. This may be less than the Air Time depending upon when you start and stop flying by sole reference to the flight instruments.

Ground instrument time

This is your time spent simulating non-VFR conditions on a simulated IFR flight in an approved flight simulator.
An instructor is expected to be with you when you are conducting IFR training in order to get credit for this ground instrument time, but if you are an IFR rated pilot you don’t need an instructor. You can be alone or with another pilot.
If you are in non-VFR conditions for the entire flight you can only take credit for up to the maximum simulator Air Time (up – down), not the total simulator Flight Time as the simulator could be paused during the simulated flight.

These are suggested guidelines or best practices for estimating and logging “instrument time” that anyone should be able to follow to meet the 6 hours and 6 approaches in 6 months rule. There are differences between US and Canadian IFR recency requirements also creates more confusion. Even approved training devices are described differently so this discussion could continue endlessly.
My conclusion is that logging “instrument time” is based on your good judgement to determine if the flight was under instrument flight conditions. It is also based on the honour system for reporting your time. Please feel free to disagree with my interpretations and discuss as needed. I hope that a “standard” interpretation will eventually be achieved through consensus.

Welcome to aviation.SE! This is a very interesting breakdown, but it doesn't directly answer the question, which is specifically about US currency using a simulator. If you aren't familiar with the StackExchange sites, you might want to check out the [tour]: this is a Q&A site, so we try to stick to direct answers, preferably based on official sources. I hope you'll stick around and contribute more!

In addition to not answering the question (note the faa-regulations tag), there are some inaccuracies: (a) dark night vfr doesn't require an IFR flight plan. (b) you only need 15 hours with a CFII for your certificate. The rest can be with a other (non-CFI) pilot, who may or may not be able to log PIC. (c) The FAA doesn't really recognize the concept of "ground instrument time", just time in an  FFS/FTD/ATD. As of April 2017, an instructor is required to log ATD time, whether or not you're rated. There is no 6 hour requirement in the US.

I had searched many questions here before posting like: "Can I fly IFR in VMC if my instrument rating is not current", etc.. The invitation to post here also said "Share your knowledge with the Aviation Stack Exchange community."  Therefore the question is answered under ground instrument time but I thought I would also share my research with instructors from other countries and even AOPA and COPA. The key point is that the regulations in the US, Australia and Canada are not harmonized and very convoluted. Because interpretations differ as you point out, your own good judgement must be used.

I ran out of characters above... .   It should end like this:
Your own good judgement must be used when interpreting the regulations.


text:
What is the procedure when being vectored for an approach and ATC allows you to pass through the final approach course?
If you are on a heading, being vectored to intercept final on an instrument approach, and it appears that you will fly through the final approach without being cleared to intercept it, what should you do?

Usually when ATC has a need to vector you across the final approach course, they will tell you about it before they do.  Something along the lines of "N1234 fly heading 230, vectors across final for spacing.".  If they don't and you see that you are getting close, you should ask them if they want you to intercept the course.
The AIM addresses this in paragraph 5-4-3 Approach Control:

(b) After release to approach control, aircraft are vectored to the
  final approach course (ILS, MLS, VOR, ADF, etc.). Radar vectors and
  altitude or flight levels will be issued as required for spacing and
  separating aircraft. Therefore, pilots must not deviate from the
  headings issued by approach control. Aircraft will normally be
  informed when it is necessary to vector across the final approach
  course for spacing or other reasons. If approach course crossing is
  imminent and the pilot has not been informed that the aircraft will be
  vectored across the final approach course, the pilot should query the
  controller.
(c) The pilot is not expected to turn inbound on the final approach
  course unless an approach clearance has been issued. This clearance
  will normally be issued with the final vector for interception of the
  final approach course, and the vector will be such as to enable the
  pilot to establish the aircraft on the final approach course prior to
  reaching the final approach fix.

The FAA Air Traffic Control Order Section 9 - Radar Arrivals also has the following directive for approach controllers:

5-9-3. VECTORS ACROSS FINAL APPROACH COURSE
Inform the aircraft whenever a vector will take it across the final
  approach course and state the reason for such action.
NOTE-
In the event you are unable to so inform the aircraft, the pilot is not expected to turn inbound on the final approach course unless
  approach clearance has been issued.
PHRASEOLOGY-
EXPECT VECTORS ACROSS FINAL FOR (purpose).
EXAMPLE-
“EXPECT VECTORS ACROSS FINAL FOR SPACING.”

TL;DR: "Trust but verify". Don't turn final unless instructed, but if you're approaching the final vector with no instruction either way, radio the tower to verify.

This is one of the most confusing areas of IFR flight.  Controllers will use many different phrases like "turn to heading XYZ... intercept the localizer" or "... join the localizer" or "... report established" etc.  Oftentimes before they will issue the clearance, which they hold back as long as possible.  They want you to turn to XYZ and then turn again onto the final approach course.  This would appear to be against their guidance but it happens quite often.  Note, you are not "cleared" for the approach yet, even though you are maneuvering onto the final approach course.  You must maintain your last assigned altitude and speeds until cleared.  To answer your question, if you are "turned to heading XYZ" but not cleared for the approach nor told to "join" the final approach course/localizer/etc., I would confirm if the controller wants you to fly through the final course (or he just forgot about you).

This answer is not correct. "Intercept the localizer" 100% every single time means that you should fly your assigned heading until the localizer comes alive, and then maneuver laterally to follow the localizer. You are correct that it is not an approach clearance and does not allow you to start descent to follow the glide path. I interpreted the question more as if you were on base and expecting a PTAC but didn't get it before blowing through final. You can expect at least two headings: A base (90º from final) and a "dogleg" to join (20º or 30º from final); this is not "against our guidance."

In addition to the other answers, there is one more thing to consider - lost comms.
This scenario here is a classic for thinking about what your actions will need to be in the event of loss of communications.
Keep track of WHY those vectors are being given to you - where are they taking you, when should you expect headings that would make sense in regards to shooting the approach, etc. Always be questioning what ATC tells you to do (and of course call them if it does not make sense). Always try to think "what comes next", this will help you prevent problems when "what comes next" is an error on either their behalf, or yours!

I think this answer adds something valuable, but on its own is not answering the question. You should consider expanding it to a full answer.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the difference between ICAO, JAA, FAA, EASA, and CAA?
There seems to be many regulation bodies across the globe, how are they different and how are they alike? Specifically, how is ICAO different from the rest?

ICAO is the International Civil Aviation Organization, a specialized agency of United Nations. It was created after the Convention on International Civil Aviation (the Chicago Convention) of 1944 was ratified in 1947. The purpose of ICAO is according to the convention:

"WHEREAS the future development of international civil aviation can greatly help to create and preserve friendship and understanding among the nations and peoples of the world, yet its abuse can become a threat to the general security; and
WHEREAS it is desirable to avoid friction and to promote that co-operation between nations and peoples upon which the peace of the world depends;
THEREFORE, the undersigned governments having agreed on certain principles and arrangements in order that international civil aviation may be developed in a safe and orderly manner and that international air transport services may be established on the basis of equality of opportunity and operated soundly and economically;
Have accordingly concluded this Convention to that end."

Within ICAO, the 191 Member States and a number of global aviation organizations work together to develop international Standards and Recommended Practices (SARPs). These SARPs are the reference for states developing their national civil aviation regulations which are legally enforcible. This is an important aspect: ICAO SARPs are not legally binding by themselves, they form the basis of national regulations which have legal status.
In this way civil aviation regulations are harmonized all over the world, with slight differences based on the actual implementation in national regulations. These local differences are then reported back to ICAO and published.

CAA is the Civil Aviation Authority. This is a generic term used in many countries, notably the UK and China. A CAA is a national regulatory body responsible for aviation. The CAA implements the ICAO SARPs in national legislation and is responsible for regulatory oversight.

FAA is the Federal Aviation Administration. As the Civil Aviation Authority of the USA, it is responsible for establishing aviation regulations in the US. These are known as FARs (Federal Aviation Regulations).
In addition to its regulatory role, the FAA is also responsible for Airspace and Air Traffic Management, maintenance of Air Navigation Facilities infrastructure and has an active role in Research and Development of aviation related systems and technologies.

JAA were the Joint Aviation Authorities, a co-operation of most European (EU and non-EU) civil aviation regulatory authorities. It was not a regulatory body itself, the member authorities were responsible for the regulation.
Originally started as the Joint Airworthiness Authorities in 1970 its objectives were to produce common certification standards for large aircraft and aircraft engines to facilitate a European aviation manufacturing industry (Airbus). Over time, the scope was extended to included aircraft operations, maintenance, licensing and certification/design standards for all classes of aircraft.
With the creation of EASA (see below) in 2002, the EU members transferred the airworthiness regulations away from the JAA. Over time, EASA became responsible for operations and licensing as well.  In 2009 the JAA system was disbanded. Only the training organisation (JAA-TO) remains, it mainly provides courses for CAA staff of European countries.

EASA is European Aviation Safety Agency. Created in 2002 by the European Commission, EASA took over the functions of the Joint Aviation Authorities of the EU countries.
The responsibilities of EASA include drafting of aviation safety legislation and providing technical advice to the European Commission and to the EU Member States, airworthiness and type certification of aircraft and aircraft parts for aircraft operating in the EU, approval of aircraft design organisations world-wide and of production and maintenance organisations inside and outside of the EU.
Since its creation, the competences of EASA have gradually expanded. This process is still going on.

Let me give you a brief description of all the authorities you mentioned (as far as I know them). I hope this will help you and answers your question.
ICAO - The International Civil Aviation Organization is was founded to set up international standards in aviation. When international air travel became more and more present (1947) there was a need have international regulations. The ICAO counts 191 member states and all have to adopt their guidelines or advice the ICAO of regional differences, which will then published by the ICAO.
FAA The Federal Aviation Administration is the aviation authority of the United States of America. It is responsible for setting up the federal aviation regulations (FAR), which are binding for all flight operations in the US. The FAA also provides air traffic control service at most control towers and all en-route air traffic control centers.
EASA - The European Aviation Safety Agency, former Joint Aviation Authority (JAA), is best described by taking both names: It's the European aviation authority. I personally (on my license it still says JAA) don't see the advantages of EASA compared to JAA. I've heard that they are aiming for a 'Single European Sky', which I would really appreciate, but it also costs a lot of money and means a bunch of new paperwork. Let's return to your question as an authority you can compare EASA to the FAA, but you need to remember that it is still an international and organization, so even if they are small, there are regional differences.
CAA - Honestly I don't even know what the abbreviation stands for, but as far as I know it's the Chinese aviation authority and a governmental organization of the republic of China. It's also Civil Aviation Authority for UK.
As you see, all of them are there to establish rules for aviation in order to make it as safe and efficient as possible. The ICAO is international and responsible for very basic - they are still complex - general rules. The other authority are making there regulations according to, or at least, based on the ICAO standards.

I've never flewn there yet and it was also no mentioned in x-thousend of EASA ATPL questions, but it will be mentioned in ICAOs doc 4444. Now we need to see if he have to look in the Asian or North American ;) Please feel free to edit this answer if you find obvious mistakes.

CAA = Civil Aviation Authority ? ie. regulatory body in a country

Obviously its a very popular abbreviation and we all are some kind of right. Regarding my part (China): http://en.wikipedia.org/wiki/Civil_Aeronautics_Administration_(Republic_of_China)

@Radu094 Insofar as the U.S. is concerned, when I was a boy in the late 1940s and early 1950s, the CAA was the Civil Aeronautics Administration. It was renamed to the Federal Aviation Agency (FAA) in 1958, which then became the Federal Aviation Administration in 1967.

ICAO was explained.
CAA is rather common name for aviation authorities in the country. Like police for law enforcement. But different countries call the same differently. Like in Russia they have name milicija for police
Here is a complete list of CAAs: https://en.wikipedia.org/wiki/List_of_civil_aviation_authorities
FAA is mega scale CAA for americans.
EASA is mega scale CAA for European union.
Small countries usually follow FAA or EASA or other big authority.


text:
How safe is IFR in class E?
In class D and E airspace, there is no separation between IFR and VFR traffic. However, most airspace in the United States below 18,500 feet MSL is class E airspace, which is exactly where non-pressurized aircraft cruise when flying IFR.
My question is not about regulation (that's perfectly clear: no separation between IFR/VFR) but I'm curious to learn how safe it actually is when cruising at 10,000 feet in VMC on an IFR flight plan while in class E airspace. I'm pretty sure many pilots will be on autopilot without taking too much notice of what's happening outside but according to the regulations the pilot is still responsible for seeing and avoiding other VFR traffic.
Are there any accident statistics on this? I have read hundreds of accident reports over the last few years and I can't remember ever seeing a report on a midair collision between IFR and VFR aircraft in class E.

No 99% will NOT be on autopilot and will be taking notice of what is going on outside. Also are you asking about in IMC, VMC, or both? Because in IMC it should still be separated because no one SHOULD be flying VFR. Even in VMC though the VFR pilots AND the IFR pilots should be looking out for each other AND the IFR pilots (or VFR with advisories) ALSO have the ADDED advantage of ATC RADAR service.

Since I'm asking about VFR/IFR separation, I'm obviously talking about VMC conditions. The question wouldn't make sense otherwise.

Well now that you edited out that 99% comment.. It's probably safer than VFR because ATC is watching too. Just because they aren't the responsible for separation doesn't mean that a second set of eyes is a bad thing. Most of the time if they see something they will say something. It seems like before you were insinuating that pilots on an instrument flight plan aren't paying attention. Complacency is always dangerous, in IFR or VFR but being on an IFR flight plan doesn't degrade safety, sorry.

The essence of my question is this: We all know that we have to watch out for other traffic when flying VFR but whether we like it or not, cruising IFR at 12,000ft gives a false sense of security and pilots pay less attention than they should.

Wrong, the problem is you are still confusing VFR with VMC. What you should be saying is: "We all know that we have to watch out for other traffic when flying VMC whether we like it or not".

You're missing my point. I know what you're trying to say but I mean that pilots flying on an IFR flight plan have a false sense of security. I know very well what the difference is between VMC/IMC/VFR/IFR.

Philippe that contradicts what you said though. Being on an IFR flight plan should not give you any false sense of security. It is the pilot's responsibility to avoid other aircraft in VMC. Being on a flight plan does not change this. ATC only separates IFR traffic from other IFR traffic (Unless it's like a Class B where EVERYONE is on a clearance). So unless you are flying where there is no VFR traffic like in a cloud or above 18k ft than you have to see and avoid. In VMC you must see and avoid anyway even if you are on an IFR flight plan.

I mean if a pilot is flying around in VMC not looking for other airplanes, or birds, or ballons, or anything else that's just a wreckless pilot. They would be just as dangerous in VFR as IFR. That kind of false sense of security is just a hazardous attitude in any place under any flight plan or not.

Consider also asking "How safe is IFR in Class G" -- for context, see https://aviation.stackexchange.com/q/94265/34686

@p1l0t -- re your last comment above-- did you really mean "reckless" rather than "wreckless"?  ; :

@quietflyer Yes this is a common misspelling.

So your question is basically "is flight under IFR significantly less safe than flight under VFR, in Class E airspace in VMC?". Presumably because the pilots will likely spend less time looking out the window in the IFR-in-VMC case than in the VFR case?

Re "I have read hundreds of accident reports over the last few years and I can't remember ever seeing a report on a midair collision between IFR and VFR aircraft in class E." -- would the business jet that collided with the sailplane near Reno NV a few years back qualify?  (I don't know whether the jet was flying IFR or VFR.)

As with any flight, "It's as safe as you make it".
The vast majority of the General Aviation fleet (piston singles) don't even have an autopilot, and the ones that do may only be single-axis units (wing levelers), so the pilot has to have some level of attention on flying the airplane.
If we assume these are the folks flying around VFR we can also generally assume they should be looking out the window the vast majority of the time (like Skip said, "Put down your iPads and do your job.")
Adding to that, "See and avoid" is every pilot's responsibility in VMC - whether operating under Visual Flight Rules or Instrument Flight Rules - if you can see, you are expected to avoid (and if necessary deal with ATC freaking out about you deviating from a clearance later).

A number of other steps have been taken to enhance safety in Class D and E airspace:
In the US FAR 91.159 prescribes specific VFR cruising altitudes, and FAR 91.179 is the IFR equivalent.
The cruising altitudes in these regulations result in an automatic 500-foot vertical separation of VFR and IFR traffic (at least between 3,000 feet AGL and 18,000 feet MSL - which is where VFR and IFR traffic are likely to be mixing in cruise. ).
Additionally IFR traffic is usually receiving radar separation services (there are some non-radar areas in the US, but not many). Nominally this separation is "IFR-to-IFR" in Class E airspace, but if a controller sees a VFR target squawking 1200 and behaving unpredictably they can vector IFR traffic around it.
Similarly if a controller sees a "primary target only" (something without a transponder, and not reporting altitude information) they will often vector IFR traffic around it until they can establish what it is and what it's doing.
Within class D airspace the level of service you'll get depends somewhat on the equipment the controllers have.  A "purely visual" tower with no radar can only give traffic advisories for aircraft they can see by looking out their window, but if they see a conflict they're going to say something. A tower with radar (either local or slaved from a nearby airport) may give VFR-to-VFR or VFR-to-IFR separation advisories based on that data as well, sometimes including advisories to aircraft inbound who are outside their airspace.

Class D with no tower may not see every plane, but they certainly should know where every plane in their airspace should be!

@rbp Funny word "should" - people often confuse it with "is", but that's not always the case (with regard to both the tower knowing where aircraft are, and the aircraft being where the tower expects) :)

In class D and E airspace, there is no separation between IFR and VFR
  traffic.

This is not a true or correct statement. In these airspace classes, there is no requirement by ATC to provide separation between VFR and IFR traffic. There must still be separation in order for flight to be safe, therefore the responsibility to maintain this separation ultimately rests with the pilots of both aircraft.
ATC is responsible for providing IFR-IFR separation in any controlled airspace. VFR pilots, for their part, are required to "see and avoid" other traffic around them. This is half of why the VFR separation minima exists; you as a VFR pilot must have sufficient visibility in all directions to be able to spot other aircraft and maintain a safe distance from them.
IFR pilots have the same "see and avoid" responsibility when operating in VMC; if ATC instructions would cause the pilot to violate separation minima with observed traffic that ATC doesn't seem to know about, the pilot receiving the instruction must respond that he cannot do as instructed because of the observed traffic, which will include a report of its general position. This is the other half of the reason for VFR separation minima; if you're not flying under direct guidance of ATC, other traffic, IFR or VFR, must be able to see and avoid your aircraft, and therefore you have to stay far enough away from clouds that you're either not a danger to anyone travelling through the cloud or around the other side, or they can see you with enough time to maneuver to avoid you if you are.
ATC may provide separation involving VFR craft in Class D/E airspace as information and workload permit. VFR traffic, while not required to maintain contact with ATC, may still do so, reporting position and altitude (if ATC doesn't have a radar fix) and receiving traffic advisories of anything ATC does know about. This doesn't relieve the VFR pilot's responsibility to see and avoid but it can help him stay aware of the presence of nearby planes. This becomes more important near major airports in dense areas of airspace, which is why Mode C veils exist; while within the veil, aircraft in Class E can be easily seen and their altitude known, even if they aren't talking to ATC. ATC can then advise IFR traffic to maintain separation.
Obviously, in IMC, pilots can't "see and avoid", therefore VFR flight is illegal and IFR flights have to trust and follow ATC instructions unless anything they might still see or know obviously indicates otherwise.

I'm pretty sure 99% of pilots will be on autopilot without taking too much notice of what's happening outside but according to the regulations the pilot is still responsible for seeing and avoiding other VFR traffic.

Well, shame on the pilots, then.  Put down your iPads and do your job.  It is "see and avoid".  Perhaps they have some automation in the cockpit to help them like TIS or TCAS etc. but it is their responsibility to avoid a collision - period.  And the old Mark I eyeball is frequently the only way to accomplish that task.
To answer your question more directly, the system works pretty well.  I don't have statistics but I do know that we would read a lot about it in the press if collisions happened regularly.

ATC isn’t going to silently sit there and watch as two targets collide simply because one of them is VFR; they are going to do whatever they can to prevent that from happening.
However, ATC can only guarantee a specific level of separation between two planes if they talking to both of them.
VFRs often do unpredictable things that mess up whatever plan ATC had to keep IFR (and VFR FF) traffic away from them. And until ATC can react with a new plan, it is possible that the actual separation may be slightly less than the (very generous) ideal, though still without any real danger.
And, as others have noted, all pilots in VMC are still expected to “see and avoid”.  But ATC will nearly always advise you of any nearby traffic and have a plan for separating you long before it is close enough to see.


text:
When conducting a circling approach, how close do you have to stay to the airport (especially with the new FAA requirements)?
I heard that the FAA recently changed how they calculate the protected area for circling approaches, and it looked quite complicated because it changes based on altitude and speed, etc.  Can you please explain how you know when to use the old -vs- new criteria, what distance you can safely use, and how to figure it out quickly when flying?

Do you have an example of an approach in which the new criteria have taken effect?

@egid No, I think that the first ones will be coming out in May.

The protected area for circling approaches is based on distance from the runway(s), and the size of that area (and your approach MDA) depends upon your aircraft's VREF or 1.3×VSO — or, if higher, your actual IAS approach speed. Here's the graphic the FAA uses to demonstrate this, along with the old protected area radii:

New circling minumums
For approaches that use the new criteria, both Aeronav and Jeppesen charts show an inverted "C" in the circling minimums section of the approach plate. This signifies that the approach uses the new, altitude-dependent protected areas. Those areas are defined in the same way as before, but with different radii.
Aeronav (NACO) format:

Jeppesen format:

As far as figuring out how large your protected area is in flight, you'll have to use the inverted "C" as a cue to look up the table that will be included in the appendices of the approach plates:

The NBAA has a more detailed description of the changes, but this basically sums it up.

The update leaves us with what is honestly a less-than-desirable system. Looking up a table to brief your circling radius in flight (or even prior to your flight) is certainly a lot less useful than simply printing the protected area radius on the minimums section. It's worth noting that feature was proposed in 2009 (p. 4) when the change was being discussed.


text:
How does aircraft navigation work?
Is it possible to briefly summarize how a commercial plane navigation system operates to a layperson (e.g. a passenger)?
The aircraft takes off, reaches a certain altitude, and then lands perfect on a runway somewhere else.
How does the navigation system affect this?
How do pilots know where to go and where to land?

Ok so I changed cos I dont unserstand.. how pilots navigate..

What does being a passenger have to do with this question?  Are you asking about if passengers are involved in navigation?  I might suggest that this question be closed as too broad, unless someone wants to take a stab at providing a general outline of the navigation system + ATC.

@PedroMorgan "How do pilots navigate?" is a really broad topic with a number of answers, I don't think we can really do it justice in an answer here, but there's two broad categories: "Look out the window at the ground, compare it to a map, and figure out where the heck you are using landmarks" (Visual Navigation), and Instrument Navigation which encompasses a lot of different techniques using radios, GPS, etc. Chapter 9 of the Instrument Flying Handbook may help you put a finer point on this query.

I think this question could be great if it were about the basics of airliner navigation. Its interesting if you've just been a passenger.

How do you navigate in your car?
Before you depart, you have a general understanding of which direction you're going.
(the supermarket is to the east of my house, about 3 miles.  I believe it will take me about 5 minutes to get there).
When you start driving, you keep your eyes open looking for information and clues.
(The yellow lines on the road denote a two-way street.  Certain landmarks are familiar and expected.)
As you proceed, you may refer to your car dashboard to make sure things are as expected
(My speed is 43MPH.   I had planned for 45MPH... pretty close.  My engine is running at 2,500 RPM, which is appropriate)
You may have advanced instruments as well
(The GPS in my car is telling me to take the next right turn, and go 1/2 mile)

Its very much the same with flying.
Pilots have pre-flight preparation where they go over a general plan.(we're going to fly north, on a heading of about 025 for 3 hours.  We'll be passing over a large lake, and landing at our destination with a city off to our left)
As they move towards the runway, they look for other airplanes, read the signs around the  runway, and refer to their maps.
(I see a sign indicating I'm on taxi-way Bravo, approaching Runway 31.  I see yellow markings on the pavement indicating I should get permission before I cross)
Once they take off, they look at their instruments to verify they're going at the expected speed, altitude and direction.
(Altitude is 35,000 feet, as planned.  Engines are at the right pressure, and speed is 440 knots, as expected)
They can look out the window for large landmarks, lakes, mountains, cities, etc.
(I'm passing by the mountain I saw on the map.  I see the river valley below me, and a large city ahead of me, just like I planned)
They may use advanced services like GPS Navigation, Radar and radio signals to help navigate.
(My GPS is showing my position and direction.  It says to turn a little to the right to stay on course.   The navigation radio says I'm 25 Nautical Miles away from the radio transmitter)
And they can talk to Air Traffic Control who can observe them on radar, and advise them to make adjustments to their course for navigation, and to stay separated from other airplanes.

If they cannot see out the window, navigation does become a bit more complicated, and reliant on Instruments, GPS, Radar and Radio.But given a very careful plan of "Fly for exactly 31 minutes, at exactly a heading of 027, at exactly a speed of 130Kts", it is pretty remarkable how close the airplane will end up to the intended destination.
That process is called Dead Reckoning.  It requires some detailed advanced planning, but isn't that difficult.   Combining Dead Reckoning with Instrument flying, and pilots can go through zero-visibility and still come out at the intended location.


text:
What is a Simultaneous Offset Instrument Approach (SOIA)?
I have heard about Simultaneous Offset Instrument Approaches (SOIA), special procedures for runways that are spaced particularly close together. How exactly do they work? What are the requirements for an airport to be eligible for the procedure? What special requirements are there for pilots flying the approach?

SOIA (Simultaneous Offset Instrument Approaches) allow airports with parallel runways that are 750 to 3000 feet apart to conduct (almost) simultaneous approaches to the two runways. At an airport, one runway uses the ILS PRM approach, while the other runway uses an offset LDA PRM approach (with glideslope). SOIA refers to the LDA PRM approach, where another aircraft is flying the ILS PRM approach ahead. PRM stands for Precision Runway Monitor, which is a high-update radar system allowing for "equivalent" safety with reduced margins.
For a PRM approach is conducted, there must be two frequencies per runway:

a primary frequency for both the transmission and receipt of instructions
a "monitor" frequency, only for ATC to transmit on

The reason for the "monitor" frequency is just in case the primary frequency is blocked and breakout instructions have to be provided. The tower controller will communicate to aircraft on both frequencies, both in normal PRM operations and when providing breakout instructions.
There is also a "monitor" controller that ensures that the two aircraft never get dangerously close together -- if such a situation occurs, the "monitor" controller overrides the tower controller on both frequencies to allow for immediate breakout instructions to be given.
Of the two approaches, the more interesting of the two is the offset LDA PRM approach. In the LDA PRM approach, aircraft fly an approach course that is 2.5 to 3 degrees offset from the runway heading, following the glideslope until the MAP (this course does not meet up with the runway threshold), at which point the pilot should proceed visually to the runway.

The MAP on a LDA PRM approach is 3000 feet away from the ILS approach course, meaning aircraft have to manoeuvre towards the extended runway centreline to be stable by 500 feet -- the less spacing between the two runways, the more aircraft will have to turn. This does mean that it is less suitable to larger aircraft, due to the manoeuvring required at higher speeds (than other aircraft), though most SOIA approaches are designed for larger aircraft as well. Before reaching the MAP, the pilot must have have:

the runway in sight;
the aircraft on the adjacent ILS PRM approach in sight; and
notified ATC that traffic is in sight.

This means that the aircraft on the ILS PRM approach should be ahead of the other aircraft. To allow time for visual contact, aircraft should be clear of cloud before the MAP, which is taken into account when deciding on the minimum cloud ceiling allowed for the approach.
The NTZ (No Transgression Zone) is a 2000 foot wide area between the two approach courses which aircraft must not enter at any time. ATC would immediately issue breakout instructions to both aircraft if an aircraft breaches the NTZ, as there would be very little horizontal separation between the two aircraft. [The FAA AIM][3] states that:

In the unlikely event that an aircraft "blunders" off its course and makes a worst case turn of 30 degrees toward the adjacent final approach course, closing speeds of 135 feet per second could occur that constitute the need for quick reaction.

Special training requirements vary from country to country. In the US, SOIA special training requirements are:

Parts 121/129/135: operator training as agreed to by FAA and educational video
Part 91 transport category: read about PRM approaches in AIM, video
Part 91 other: read about PRM and SOIA approaches in AIM, video

An example of a SOIA approach is the SFO runway 28R LDA PRM approach:

fabulous answer


text:
Do Special MEA altitudes and routes in Alaska require WAAS?
Low Enroute IFR charts in Alaska (and maybe elsewhere?) have blue routes and MEAs for GPS equipped aircraft, such as T266, 7000G MEA, below. Is an IFR GPS sufficient to use these, or is WAAS required?

I thought WAAS was only required for precision (GPS) approaches

The answer appears to be yes. The FAA document that describes this rule says:

TSO C145a and TSO C146a GPS WAAS navigation systems are authorized to
  be used as the only means of navigation on Federal airways and other
  published ATS routes in lieu of ground-based navigation aids in Alaska

The IFR chart legend says:

To utilize these routes aircraft will need to be equipped with IFR
  approved Global Navigation Satellite System (GNSS). In Alaska,
  TSO-145a and 146a equipment is required.

And both TSO C145 and C146 specifically mention WAAS in their titles.

Live and learn :-)


text:
Are you required to report leaving an altitude if you have been given a crossing restriction?
If ATC gives me a clearance to cross a fix at a specific altitude or a descent at pilot's discretion and I read back the clearance, is my readback considered the report specified in the AIM, or do I have to inform them when I actually start down?

AIM 5-3-3. Additional Reports
a. The following reports should be made to ATC or FSS facilities
  without a specific ATC request:
1. At all times.
(a) When vacating any previously assigned altitude or flight level for
  a newly assigned altitude or flight level.
...

An Example - Say that a pilot is flying at flight level 340 100 NM from Wilmington VOR and gets the following clearance:

ATC: N1234, cross Wilmington VOR at flight level 240
  Pilot: Cross Wilmington VOR at flight level 240, N1234

or

ATC: N1234, descend at pilot's discretion to flight level 240
  Pilot: Descend at pilot's discretion to flight level 240

It's a little early to start down since we want to stay high for fuel efficiency, so the pilot decides to wait until 30 NM from the VOR to start down.  When they reach that point, are they required to inform ATC before starting the descent or are they considered to have reported the descent earlier?
I hear pilots that do it both ways and each camp is pretty emphatic about it....

They are usually giving you a descent clearance before that, but could be at pilot's discretion, in which case I am not exactly sure. We're talking IFR right, and not VFR who's about to enter a B or something?

@roe: Yes, IFR.  It is actually very common for them to have us cross a fix (or "cross 40 miles North of" a fix) at a specific altitude and leave it up to us as to when to start down.  The same question can be asked of a descent a pilot's discretion however, and I will add that to the question.

It certainly can't hurt to call again, and it adds to the level of safety. In flying, I choose pedantry over brevity any day.

@JonathanLandrum: I agree that it can't hurt, but I am trying to determine the actual requirement.  Sometimes, in very busy airspace it can be hard to squeeze this call in, and if it isn't required I would sometimes skip it.  I also get varied reactions from controllers, sometimes feeling like I am bothering them with a call that isn't required.

I’ve been a controller in both the en route and terminal environments. While I agree with your interpretation of the AIM that it appears to be required, reorting leaving your altitude is not expected and I did not usually hear it done.
To give the perspective from the other side of the mic, an altitude crossing restriction is functionally equivalent to a pilot’s discretion climb/descent to that point. ATC will protect all altitudes between your current one and the restriction, and you are free to hit that restriction as you see fit (barring the violation of other regulations, of course). You may not return to a previously vacated altitude as part of your climb/descent, but you may level.
You are also correct in that it can tie up the freq as an “unnecessary” call, but if you are unsure, better to over inform rather than under inform.
Incidentally, very few controllers read the AIM. The 7110.65 takes precedence, and I’ve actually had a supervisor yell at me for referring to the AIM once. I know, I know.
So, legally, it should be done.  In practice? Unnecessary and I preferred that it not be done when I was working.

It's great to see a controller answer this, welcome to our site!

Short Answer
No, but you should.

Long Answer
I see two questions which I will answer—or attempt to answer—each in turn. Firstly, from the title:

Are you required to report leaving an altitude if you have been given a crossing restriction?

And secondly:

If ATC gives me a clearance to cross a fix at a specific altitude or a descent at pilot's discretion and I read back the clearance, is my readback considered the report specified in the AIM, or do I have to inform them when I actually start down?

To the first question: no, you are not required to report leaving any altitude. Those items that must be reported are listed in 14 CFR 91.183, including:

(a) The time and altitude of passing each designated reporting point,
or the reporting points specified by ATC, except that while the
aircraft is under radar control, only the passing of those reporting
points specifically requested by ATC need be reported;
(b) Any unforecast weather conditions encountered; and
(c) Any other information relating to the safety of flight.

Leaving an altitude is clearly not listed here. However, as you note, the "leaving an altitude report" is found in the the Aeronautical Information Manual (AIM):

5-3-3. Additional Reports
a. The following reports should be made to ATC or FSS facilities
without a specific ATC request:

At all times.

(a) When vacating any previously assigned altitude or flight level for
a newly assigned altitude or flight level.

When the FAA means for something to be mandatory, the word "must" is used in regulation. However, the FAA's Plain Language Program Manager helps us understand the use of "should" as "meaning optional but implies that the writer recommends and advises the reader to use that option." Moreover, given that the AIM is itself not regulatory, we can interpret the plain language of the above excerpt at face value: the report should be made, but is not mandatory or required.
Though optional, the report is generally a good idea, recommended, advisable, and in keeping with best practice. You should give that report, even if the pilot flying gives you a funny look. I too hear some pilots give the report, and I find there are perhaps more that omit it. I have come to consider the making of that report to be one of many marks that indicate disciplined airmanship.
From a more subjective standpoint, I acknowledge that there may be valid reasons to omit the report. Indeed some jurisdictions explicitly do not require the report in radar environments unless requested by ATC. From personal experience, controllers generally seem uninterested in a vacating altitude report.
But, the FAA says you should make the report.

To the second question: no, probably not. If you readback a clearance for descent at pilot's discretion, that is a clearance readback, not a vacating altitude report. Moreover, if you receive that clearance and then commence the descent at some other time, the readback is not satisfying the "when" part of "when vacating any previously assigned altitude".
In your specific example of a descent clearance received and readback 70 NM prior to commencing descent, that readback would not comprise a report made when vacating FL340.

AIM 5-3-3. Additional Reports
a. The following reports should be made to ATC or FSS facilities
  without a specific ATC request:
1. At all times.
(a) When vacating any previously assigned altitude or flight level for
  a newly assigned altitude or flight level.

Emphasis mine.  The phrase 'when vacating' is pretty unambiguous.
Edited answer to better respond to your comment. Previous answer below.

The AIM explains the purpose of readbacks in 4-4-7b:

ATC Clearance/Instruction Readback.
  Pilots of airborne aircraft should read back those parts of ATC clearances and instructions containing altitude assignments, vectors, or runway assignments as a means of mutual verification. The read back of the “numbers" serves as a double check between pilots and controllers and reduces the kinds of communications errors that occur when a number is either “misheard" or is incorrect.

Although this is stretching, I find it easier to believe that a readback does not qualify and you should call again when you actually start down.
Furthermore, the AIM references FAR 91.183, which says that the pilot of an aircraft operating under IFR must report as soon as possible

(c) Any other information relating to the safety of flight.

If you figure altitude changes qualify as safety of flight information, it's probably not a bad idea to make a second radio call.

So the answer is "no", it's not required. The AIM says you should notify ATC, but there is no requirement to do so.

The call is not required and I would even argue it’s undesired by the indifference you will hear back from the controller.  When issued a pilot discretion descent or crossing restriction descent, you have effectively been given a block altitude of clear airspace as your new “assigned altitude” with the caveat that you cannot return to an altitude once leaving it.  At no point in this descent are you vacating your assigned altitude block, so the note in the AIM does not apply to this situation.  Get it?  Your new altitude assignment is a block altitude bounded by your current level through the level off restriction, and you have already read back that clearance, so you’re not vacating your altitude assignment when you start the descent.  You can make this call if you have doubts about your clearance or it just makes you feel better, but it’s certainly not required.  I disagree that making the call is “disciplined airmanship” and it just shows you don’t understand that your currently assigned altitude is a block vice a single level, and that’s why you’ll get funny looks and annoyed responses.  Let’s  say you make the call and then level off halfway through your discretionary descent - which is allowed - are you going to make the call again when you resume the descent?

Absolutely report leaving the previously assigned altitude.  And this applies equally when receiving a clearance "cleared for the approach."  I recall once being cleared for an approach from a holding pattern.  Planes were stacked above me.  I wanted to maintain my holding altitude until over the holding fix.  Thus I did not say, "Leaving 2000 feet." I left that altitude quickly when approach control told the plane above me to descend to 2,000 feet.

You answered in the affirmative, meaning, I understand, that a pilot is indeed required to report vacating. Can you provide a reference?

In the US...
Under radar contact the leaving level phraseology is not expected nor needed by ATC. What they do need is you tell them what altitude you are at (plus climbing/descend to) when you check in.
The leaving level is mandatory when flying without radar contact.
But AIM tells us to do it anyways.
I listened in nearly 100 hours of recent flying (Premier 1 Driver on youtube), and he never uses leaving level, ATC never asks him to. Other pilots don't use it either.
It is confusing because the AIM teaches us to do it, as well as my instructor did when I got my IFR rating. And it used to be SOP even in radar contact a long time ago.
I'm PPSEL IFR USA. But I haven't flown in 15 years.
Ok, I can't add a comment to user331's answer which conflicts with mine, so let me ask here.
Go out and listen to tens and tens of hours of ATC comms.
You will find out that what's being done is reading back the descent clearance.
When the descent is immediate that creates the impression that the purpose of the readback is to confirm leaving that level, but when its a PD or a descent with a constraint that can be delayed, you will find the pilot doesn't come back and informs leaving later.
So what I said matches real world actions.
If ATC wanted us to report actually leaving the altitude, they would be chastizing every body for not reporting when the altitude change is delayed. The practical reason is simple, under radar contact ATC can see your descent on radar. Without radar contact its a very different deal, ATC cannot see your transponder altitude so they cannot confirm you are descending, vacating your original altitude so actual confirmation is required.


text:
Is an alternate airport always required when flying internationally?
When flying a domestic US flight, an alternate airport is not required unless the weather is below specific minimums at the destination.
What are the rules when on an international flight plan and leaving or returning to the US?
In larger airplanes, carrying the extra fuel required to fly to an alternate (which costs money) is a little silly when there isn't a cloud in the sky and there are multiple airports in the vicinity of your destination that can be easily reached with your already required 45 minute IFR reserve.  We should always have a plan for when things go wrong, but if it can be done safely, is it legal to file an international flight plan without an alternate?

To specify it a little: FARs doesn't require you to have an alternate if an instrument approach procedure is available at your destination and the forcasted weather at your destination between one hour prior and one hour after xour estimated time of arrival is 3SM or more visibility and ceiling 2000 feet or higher - VFR (VMC).

@Falk I'm asking about international regulations though, not just FAR's.  :)

I just wanted to add this for those who don't know and wonder about FARs - like I did a few weeks ago ;)

There are other reasons why runway might become unavailable, for example an accident. What are you going to do than if you don't have fuel reserve to fly to alternate?

@JanHudec Fly to one within the 45 minute reserve!  As I said in the question: "We should always have a plan for when things go wrong, but if it can be done safely,.."

Related question: No alternate destination when flying from Oakland to Honolulu: is that correct?

As a simple anwer, no, it is not always required.
ICAO Annex 6 4.3.4.3 Destination alternate aerodromes

For a flight to be conducted in accordance with the instrument flight
rules, at least one destination alternate aerodrome shall be selected
and specified in the operational and ATS flight plans, unless:
a) the
duration of the flight and the meteorological conditions prevailing
are such that there is reasonable certainty that, at the estimated
time of arrival at the aerodrome of intended landing, and for a
reasonable period before and after such time, the approach and landing
may be made under visual meteorological conditions; or
b) the
aerodrome of intended landing is isolated and there is no suitable
destination alternate aerodrome.

Also, there are similar descriptions for the requirements for take-off and en-route alternates.
EASA adds aditional requirements in OPS 1.295 in the form of maximum flight time of 6 hours and at least two runways at the airport, but it is still allowed. I can only assume FAA does a similar thing

I think the answer here is "It depends on the rules of the countries whose airspace you will be using" -- For example if I'm going in to Canada from my home base in New York I would need an alternate airport if either the US or Canadian regs required one.
Of course as Lucas pointed out what is legal and what is prudent aren't always the same: you probably want to have an alternate airport in mind for every flight (VFR or IFR), and you may want to list them on your flight plans so ATC has the information (even if it's not visible to controllers it can be used for Search & Rescue purposes and the like if the worst should happen).
Similarly you probably want to have multiple realistic diversion options in mind along your whole route, just to have all your bases covered.

You all give some nice explanation but to answer the actual question, no, there are operations, where no alternate is required.
Some really remote airports which are right in the middle of nowhere are referred as isolated airports. If you are going to these fields, no alternate will be required but you are required to take a lot of extra fuel (EASA requires the fuel you would consume while holding for 2 hours in 1500 feet above the destination). I chose this example because I know that this is pretty much the same all around the world, but different authorities also approve some other flights without having an alternate. See the FAA, or even the very strict EASA which doesn't require an alternate if independent runways with instrument approaches and sufficient weather information for the destination, forecasting 'good' weather are available.
It does not depend on the state you are going to, anyway this state has to except your operation first, but on the state of the operator. The exact procedures saying for which kind of operation you need which amount of alternates, where they must be located, what minimum equipment they need to offer and how much fuel you need to take is written in the operations manual, which is at least as restrictive as the rules of the state of the operator.

Do you know if ICAO allows flights without an alternate?  Also, do you have a reference for the EASA not need one in some cases?

I'll remember the EASA stuff from my flight training. I'm not really familiar with the suggestions of the ICAO regarding this because they don't really affect me directly but for sure EASA rues and FAR are based o  these suggestions.

You maybe like to refer to page 23 of this document: http://www.easa.europa.eu/agency-measures/docs/opinions/2012/02/Part-SPO%20IR%20(Opinion%2002-2012).pdf

@Falk: Could you perhaps edit the link into the answer? Though I still don't see the exact requirement you mention (fuel for holding for 2 hours in 1500 AGL) there.

Sorry, I would appreciate to help you but curently I only have my mobile to go online, which makes things difficult. That's the final reserve fuel required by EASA when flying to an isolated aerodrome.

This isn't very clear, at least based on the published information that I could find. The AIM 5-1-9 says:

Use of FAA Form 7233-4 is recommended for domestic IFR flights and is
  mandatory for all IFR flights that will depart U.S. domestic airspace.

7233-4 is basically an ICAO flight plan form, and the AIM says this about the alternates:

(b) Alternate and 2nd Alternate Aerodrome (Optional)

But it isn't clear - to me - if the word "optional" applies here to the entire line, or only to the second alternate.
Interestingly, the AIM also says this:

NOTE- Although alternate airport information filed in an FPL will be
  accepted by air traffic computer systems, it will not be presented to
  controllers. If diversion to an alternate airport becomes necessary,
  pilots are expected to notify ATC and request an amended clearance.

That could mean that both the alternate and second alternate are optional, since there would be no point in requiring information that isn't even visible to ATC. However that's a guess and could easily be wrong (the data could be useful in other ways); I couldn't find any guidance on this on ICAO's site either. And airlines probably have their own internal rules on identifying and filing alternates as well.
A flight returning to the US would almost certainly use an ICAO flight plan, so whatever the correct interpretation of "optional" is here, it would apply to those flights too.
Anyway, regardless of what the regulations say, filing alternates does have the significant benefit of 'forcing' you to plan and review your route more carefully.

Think about why you have to choose an alternate airport. The most important thing to remember about alternates is that they will only be used when the weather is bad.  The destination airport has gone below IFR minimums, and you are now relying on your alternate airport to get safely back on the ground.  The planned flight has essentially been aborted, and a back-up plan of action is required.
So even if the rules do not dictate that you should get an alternate airport, it's best to have one or more in case something goes wrong. During an emergency the last thing you want to do is get a chart and start finding an airport.

I agree, but do we have to file one for every international flight?

Better have alternates in mind all along your flight path.  I've had to divert part way to a destination due to buildups we could not get around/through...

Well, the most important thing to remember about alternates is that they may need to be used even under other circumstances than bad weather. A blown tire is often enough to close a runway for an hour or two and if the aircraft just ahead of you becomes disabled on the only available runway...


text:
What are the weather requirements to log an instrument approach?
So let's assume that I'm the sole manipulator of the flight controls in an aircraft in which I'm rated and that I fly an instrument approach.
What weather does the FAA require (assuming that I'm not wearing a view limiting device) in order to log the approach for currency requirements?  For instance, if I am cleared for an ILS in visual conditions, can I log it?  What if I start the approach in the clouds and break out at 1,500 ft and continue the approach?  1,000 ft?  Before/after the outer marker?  150 feet above minimums?  I think that you get the idea....

Update with relevant info from InFO 15012 (I'm skipping the FTD portions as not relevant to the question, and including the simulated parts but italicizing them as they're useful but also not relevant). Translated, you can log an approach when:

it is flown solely by reference to instruments, and

it is flown in IMC, and Actual Instrument time is logged, or
the PF is using a view-limiting device, and Simulated Instrument time is logged;

you must fly the complete approach from a feeder or the IAP (or Vectors to Final) and remain established the whole time. This means you fly initial, intermediate, and final segments, unless you're on vectors.

if flying the approach in IMC, you may log an approach that:

continues to DA/DH or MDA in actual, or
becomes VMC before DA/DH or MDA, provided you are inside the FAF.

if simulating the approach, you must continue to the DA/DH or MDA under the hood (or other VLD);

That's pretty much it. Here's the source text:

A pilot may log an IAP for currency or training when the pilot
  accomplishes the IAP in accordance with the following conditions:

When conducted in an aircraft, flight simulator, flight training device, or aviation training device, the pilot must operate that
  aircraft or authorized training device solely by reference to
  instruments [§ 61.51(g)(1)];
When conducted in an aircraft, flight simulator, flight training device, or aviation training device, the pilot must be established on
  each required segment of the IAP to the minimum descent altitude (MDA)
  or decision altitude/decision height (DA/DH);
When conducted in an aircraft simulating instrument flight conditions, a flight simulator, a flight training device, or an
  aviation training device, the simulated instrument meteorological
  conditions (IMC) must continue to MDA or DA/DH; and
When conducted in an aircraft, the flight must be conducted under actual or simulated instrument flight conditions [§ 61.51(g)(1)].
When conducted in an aircraft maneuvering in IMC, and the aircraft transitions from IMC to visual flight conditions on the final approach
  segment of the IAP prior to or upon reaching MDA or DA/DH.

Previously, I linked to Rod Machado, who cites the FAA in an answer to this that I quite like, and which was pretty frequently referred to when I was instructing at a flight school:

Once you have been cleared for and have initiated an instrument approach in IMC, you may log that approach for instrument currency, regardless of the altitude at which you break out of the clouds.

...but this is not the latest knowledge anymore. It's functionally pretty close to the InFo rules for IMC approaches, though!

Here is a link to a website that discusses this issue in more detail with some more history on the FAA's views that I like:  http://midlifeflight.com/flying-faq/faq-instrument-procedures-currency/

Nice! Thanks. Might take another pass at the answer tomorrow.

That is similarto my personal rule - If its IMC when I cross the FAF, its an actual approach.

Hmmm... so how do you maintain currency in a place with perpetually good weather?

@abelenky Simulated IFR approaches! This question was specifically about approaches in actual.

The FAA has recently come out with an InFO 15012 describing in detail what constitutes a loggable approach.
Logging Instrument Approach Procedures
Portions of InFo 15012

Discussion: Section 61.57(c)(1-5) permits a pilot to use one of four
  methods to conduct and then log IAPs:

Actual instrument flight conditions flown in an aircraft;
Simulated instrument flight conditions, using a view-limiting device, flown in an aircraft with a safety pilot;
Simulated instrument flight conditions conducted in any FAA approved:
Flight Simulator/Full Flight Simulator (FFS), Flight Training Device (FTD), or Aviation Training Device (ATD),
A combination of methods 1 through 3 as prescribed by § 61.57(c)(4), or (5).

A pilot may log an IAP for currency or training when the pilot
  accomplishes the IAP in accordance with the following conditions:

When conducted in an aircraft, flight simulator, flight training device, or aviation training device, the pilot must operate that
  aircraft or authorized training device solely by reference to
  instruments [§ 61.51(g)(1)];
When conducted in an aircraft, flight simulator, flight training device, or aviation training device, the pilot must be established on
  each required segment of the IAP to the minimum descent altitude (MDA)
  or decision altitude/decision height (DA/DH);
When conducted in an aircraft simulating instrument flight conditions, a flight simulator, a flight training device, or an
  aviation training device, the simulated instrument meteorological
  conditions (IMC) must continue to MDA or DA/DH; and
When conducted in an aircraft, the flight must be conducted under actual or simulated instrument flight conditions [§ 61.51(g)(1)].
When conducted in an aircraft maneuvering in IMC, and the aircraft transitions from IMC to visual flight conditions on the final approach
  segment of the IAP prior to or upon reaching MDA or DA/DH.

I saw this, and thought that I needed to add the info here.  Thanks for beating me to it!

Having a "plain language" summary would make this answer much better though.  ;-)

I believe the new guidance now allows you to count a simulated instrument approach if you have to break off for traffic after the FAF.

14 cfr 61.51
(g)Logging instrument time.
(1) A person may log instrument time only for that flight time when the person operates the aircraft solely by reference to instruments under actual or simulated instrument flight conditions.
(2) An authorized instructor may log instrument time when conducting instrument flight instruction in actual instrument flight conditions.
(3) For the purposes of logging instrument time to meet the recent instrument experience requirements of § 61.57(c) of this part, the following information must be recorded in the person's logbook -
(i) The location and type of each instrument approach accomplished; and
(ii) The name of the safety pilot, if required.
(4) A person can use time in a flight simulator, flight training device, or aviation training device for acquiring instrument aeronautical experience for a pilot certificate, rating, or instrument recency experience, provided an authorized instructor is present to observe that time and signs the person's logbook or training record to verify the time and the content of the training session.
So back to when can the approach be logged, the way I read it, 61.51 (g)(1) would be actual instrument if the flight at that point was conducted solely by reference to instruments.  It could be 10 miles in haze or 1 mile in haze.  If your primary reference is the instruments, then it is instrument time.
Keep in mind that instrument flight can be conducted even though the weather conditions are better than VFR minima.  For example, at night with no ground reference, over water, in haze, in snow or precip.
So my short answer, is that the actual weather at any point is secondary, and the primary criteria is whether you are operating solely by reference to instruments.  And I would add that if you are in and out of clouds or precip, or smoke or whatever, you need to fly by instruments because your outside reference is unreliable.  You can still scan outside the plane, even though you rely on the instruments.
EDIT to address comment by original poster:
What weather does the FAA require (assuming that I'm not wearing a view limiting device) in order to log the approach for currency requirements? The issue is not the weather but how you are conducting the flight, and when you fly solely by reference to instruments, you may log it as instrument time.
For instance, if I am cleared for an ILS in visual conditions, can I log it?
It depends upon what reference you are using to fly.  If solely by instruments, then you can log it.
What if I start the approach in the clouds and break out at 1,500 ft and continue the approach? 1,000 ft? Again, it depends upon what your reference for flight is.  If instruments, then log the time.  If you break out at 1000 ft, you might still be in non-VFR conditions, depending upon the airspace, etc.
Before/after the outer marker?
What is your flight reference?
150 feet above minimums?
What is your flight reference?
I think that you get the idea....
Not really.  The regs are reasonably clear in that they say you can log when you fly by instrument reference.  Just because you head pops out of a cloud does not mean you are not flying by instrument reference.
To editorialize, I would be careful looking at FAA Letters of Interpretation. A local pilot did exactly that, and had his ATP and CFI revoked.  Look at the regs.  Because the regs, not some lawyers opinion on a given day, is what the NTSB will be applying at your hearing.

Okay, so change my question to read "when operating solely by reference to instruments" instead of "what weather" and there is still a lot of ambiguity about when that's required (more specifically, what about the situations in the last paragraph of the questions?).